In fluid analyzing systems where it is necessary to meter quantities of fluid, it is also necessary that these quantities be fed to the analyzing system in a positive, non-turbulent manner and as discrete increments of precise volume. In a typical analyzing system, a number of different reactants of precise quantity are also required for mixing with the fluid being analyzed to permit the appropriate quantitative chemical analysis of the fluid. An analyzing system of this type is disclosed in the copending application Ser. No. 501,875, filed Aug. 30, 1974, entitled "Liquid Analysis System" and now U.S. Pat. No. 3,999,545.
In such a system, the metering of the fluid and reactants is controlled by an air operated metering valve device. This device includes a tubular housing and a complementary shaped rotor valve plug mounted within the housing. The wall of the housing is provided with a plurality of passageways extending diametrically therethrough. The rotor is also provided with diametrically extending passageways. The passageways through the housing are divided into two sets, one of which is aligned with the passageways through the rotor when the rotor is in a filling position. These are the filling passageways and include an inlet and outlet on the diametrically opposite walls of the housing. The other set of passageways through a housing are aligned with the rotor when the rotor is in a dispensing position. These are the dispensing passageways. They also include an inlet and outlet on diametrically opposite walls of the housing but are aligned at right angles to the filling passageways. In the filling position of the metering device, fluid is fed through the filling passageways of the housing to fill the rotor passageways. As the rotor rotates, the charge of liquid in each rotor passageway first becomes isolated from the housing filling passageways; and then as the rotor rotates 90.degree. to the dispensing position, they align themselves with the dispensing passageways of the housing. The inlet of each dispensing passageway in the housing is connected to a source of air pressure so that when the rotor is properly positioned, air will force the fluid in the rotor passageways out through the outlets of these passageways.
In order to smoothly and gently dispense each quantity of fluid from each of the rotor passageways and in order to do this both completely and immediately upon the rotor reaching the dispensing position, the pressure of the driving air must be carefully regulated. In a typical metering device, there may be four or five rotor passageways and a single source of compressed air used to supply air to each of the inlets of the dispensing passageways of the housing. Also, the air is supplied to these inlets continuously so that it is immediately available upon alignment of the rotor passageways with the dispensing inlets. The pressure at which the air required is on the order of 7 to 9 inches of water column. An excessive pressure will not provide the desired gentle, smooth flow. Also, an excessive pressure can cause malfunction of the metering device of this construction. This is so since the air is being continuously supplied through the inlets of the dispensing passageways and is only blocked by the wall surface of the rotor when the rotor passageways are out of alignment with these inlets. An excessive pressure will tend to cause leakage of air along the interface of the rotor and housing. This can result in further disruption of the liquid filling cycle and the proper containment of the liquid within the rotor passageways.
In order to avoid these problems, the air should be continuously supplied to the metering device at a low pressure and generally at the same pressure that will be required for forcing the liquid in each rotor passageway through the outlets of the housing dispensing passageways. However, with a single source of air supplying all of the inlets of the dispensing passageways of the housing, the demand consumption of the metering device is greater than the rate of flow of air from the source. This excess demand consumption appears even if the total cross-sectional size of the individual inlets in the housng is equal to the cross-sectional size of the conduit supplying air from the source. It is even more evident where the total size of the housing dispensing inlets is greater than the size of the conduit supplying air from the source. In any event, this excess demand consumption may require the use of a restricting device in the conduit system to avoid a high pressure system during the nondispensing position of the rotor and to, at the same time, provide the required pressure for dispensing. Restriction devices used in this way generally require continuous monitoring to assure that the air is being properly supplied to the metering device for each position in the rotor.